Microscopy Methods for Imaging Magnetic Grains in Diogenite NWA 5480

Introduction

Diogenites are basaltic meteorites which likely originated from the planetesimal 4 Vesta. While Vesta has been studied much in regard to its composition, elements of the body's geologic history remain unknown. If Vesta at any point in its history had a geomagnetic dynamo (such as the one that drives Earth's magnetic field), small (~10 μm) magnetic mineral grains in hot magma/rocks would have been magnetized in proportion to the field strength. After ejection from their parent and a long journey to the University of Rochester, these magnetic mineral grains can be studied through a variety of experiments to quantify the strength of the ancient magnetic field they were magnetized in.

Crucial to interpreting the data from paleomagnetic experiments are the identities of the magnetic minerals within the meteorites. In addition, the validity of the experimental methods hinges on the domain state of the magnetic grains. In other words, these experiments require single-domain (SD) magnetic grains, which contain unidirectional magnetization (multi-domain (MD) grains contain regions, or domains, with differently oriented magnetizations).

Using several microscopy methods, I located, imaged, and characterized magnetic mineral grains in a thin-section sample of the diogenite NWA 5480, and tentatively demonstrated that they are single-domain.

Profilometry (Ambios XP-200 Profiler)

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With the help of engineer Sergey Korjenevski, course profilometry was taken for our sample. This method works similarly to atomic force microscopy (described below), but allows larger, rougher areas to be scanned at the expense of speed and resolution. The data pictured show that our sample sits roughly 15-20 μm above the epoxy substrate it was polished in. However, the ~100 μm windows seen in the atomic and magnetic force microscopy (AFM and MFM) below only encapsulate a small portion of this topography.


SEM Imaging (Zeiss Auriga CrossBeam SEM-FIB)

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Two types of scanning electron microscope images were taken of the whole thin section sample, as well as micron-scale magnetic grains therein. The secondary electron (SE) images on the left show scratches left from polishing, as well as a bumpy topography that is probably an artifact of the electron beam (when this picture was taken, the sample had been in the SEM multiple times).

While the SE images are crisper, the backscattered electron (BSE) images say more about the composition of the grain imaged. They do not show the same topography, but the brightness of the two grains at the top of the green box indicates a higher abundance of a heavy element such as iron, since heavy elements will "backscatter" more of the incident electrons. The invisibility of the bumpy topography in the BSE images is probably due to a deeper interaction volume for BSE's, which indicates that the bumps are not large. Below are descriptions of energy dispersive x-ray analysis done on this region, atomic force microscopy done on multiple regions, including the one highlighted in red, and magnetic force microscopy done on the regions including the one highlighted in green.

Scale bars on the top images are 100 μm on the upper images, and 1 μm on the lower images. Click for closer view.


Energy Dispersive X-Ray Spectroscopy (EDAX and Zeiss Auriga SEM)

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X-ray microanalysis provides elemental maps and abundances for SEM samples. As a sample (or a region on a sample) is bombarded with electrons, its constituent atoms will emit x-rays distinctive of their elemental spectra. These can be read by an energy dispersive spectrometer (EDS). The SEM at the University of Rochester nanocenter uses an EDAX EDS.

Using the EDAX EDS, x-ray spectra were taken for each of the grains in our ~15 μm locality. Despite the proximity of the grains, they each have a unique elemental makeup, as shown in the spectral plots to the right, and the elemental maps at the bottom. Unsurprisingly, it was the two small, iron rich grains that were able to be imaged usin MFM (below).

Click for closer view.


Atomic Force Microscopy (SOLVER NEXT AFM platform)

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Atomic force microscopy is done by moving a sample (with piezoelectric crystals) underneath a probe tip, usually vibrating the probe to produce intermittent contacting and reduce damage to the tip and the sample. As the probe tip moves up and down with the sample topography, a laser beam is deflected off the probe arm to strike a detector, which tells the height of the probe. From this a topological map of the sample is created.

We used AFM to image our sample. The top image, highlighted in red, is of the same region highlighted in red in the SEM images. While precise location with the AFM is difficult, we were able to find the magnetic grain of interest by the last two pictures shown here. While it's topolographical shape is not clearly the same as the shape seen in the SEM images, the general location and the presence of a square pattern of bumps believed to be created by an electron beam raster indicate that this is indeed the locality imaged by SEM.

The top three scans are 110 μm wide, and the bottom two are 30 μm and 10 μm wide, in that order. The sample in these images was rotated by about 200° from the SEM images. Click for closer view.


Magnetic Force Microscopy (SOLVER NEXT AFM platform)

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Magnetic force microscopy works on much the same principles as AFM does; the primary difference is the use of a magnetic probe. Prior to imaging a region with MFM, an AFM image is taken (the two left-most images). The sample is then pulled slightly away from the probe and scanned once again. On the second run, the phase of the probe's vibration is changed due to passing over differently magnetized regions, and as shown in the images on the right, sufficiently magnetic regions can be imaged.

The images shown here display progressively closer scans of the magnetic grain on our sample, and culminate in a close view of the two most iron rich particles imaged in the green-highlighted SEM picture.

Raster sizes are 110 μm, 40 μm, 40 μm, 19 μm, and 8 μm, in that order. Click for closer view.


Conclusions

Micron-sized inclusions were isolated in NWA 5480. Through energy dispersive x-ray analysis, they were shown to be iron rich, and varying in elemental composition. Two of these grains were located with AFM and imaged with MFM, demonstrating their magnetization. The image indicates that they are single-domain. Further work can include magnetizing the sample at high temperature in a known field with the possibility of imaging weaker magnetic grains.

Special Thanks

A special thanks to Sergey Korjenevski for guidance and help using the profilometer and AFM/MFM equipment, and to John Tarduno for the use of his lab’s sample. The help of both made this project possible!

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